Abstract
Many port wine stains (PWS) are still resistant to pulsed dye laser treatment.However, anecdotal information suggests that multiple-pulse laser irradiationimproves patient outcome. Our aims in this note are to explain the underlyingmechanism and estimate the possible thermal effects of multiple pulses invascular structures typical of PWS. Based on linear response theory, thelinear combination of two thermal contributions is responsible for the totalincrease in temperature in laser irradiated blood vessels: direct lightabsorption by blood and direct bilateral thermal heat conduction from adjacentblood vessels. The latter contribution to the increase in temperature in thetargeted vessel can be significant, particularly if some adjacent vessels arein close proximity, such as in cases of optical shielding of the targetedvessel, or if the vessels are relatively distant but many in number. We presentevidence that multiple-pulse laser irradiation targets blood vessels that areoptically shielded by other vessels. Therefore, it may be a means of enhancingPWS therapy for lesions that fail to respond to single-pulse dye lasertreatment.
Highlights
We have previously described a three-dimensional reconstruction of a port wine stain (PWS) which consisted of clusters of small (10 to 50 μm) blood vessels within 0.5 mm of the epidermal–dermal junction (Smithies et al 1997)
We have shown that in response to pulsed laser irradiation the temperature of a targeted blood vessel can be significantly increased if adjacent blood vessels are heated simultaneously
This linear response mechanism of propagation of thermal heat conduction from the concentric rings of blood vessels to the central vessel predicts a continuous increase of (a) the maximum temperature at the end of a laser pulse and (b) the same increase in temperature of the dermal volume. The latter dermal heating implies that the concept of selective photothermolysis becomes at risk at the dermal level. These predictions may explain Dierickx et al’s observations that ulceration could occur at the site of irradiation if epidermal cooling was not applied, and good PWS blanching with multiple pulses at lower radiant exposures than with single pulses if conductive cooling was applied (Dierickx et al 1995a)
Summary
We have previously described a three-dimensional reconstruction of a port wine stain (PWS) which consisted of clusters of small (10 to 50 μm) blood vessels within 0.5 mm of the epidermal–dermal junction (Smithies et al 1997). This particular area of the (large) PWS was resistant to single-pulse dye laser treatment, possibly as a consequence of two synergistic factors: reduced light fluence available for the deeper dermal vessels and, reduced absorption of light due to optical shielding from the more superficial vessels; and strong thermal cooling of the vessels in the clusters because of their small size.
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